How do you sink piles through an old rubbish dump full of potentially hazardous asbestos waste, where no spoil can be brought to the surface and where noise and vibration are discouraged? The answer, says foundation contractor Bachy Soletanche, is to develop a new piling technique.
The firm's cased rotary dispalcement piling system is being used for the first time on the Channel Tunnel Rail Link's Medway bridge in Kent. The technique is installing some four dozen large diameter piles through the 20m deep tip using a cone shaped screw to gently force aside the asbestos laden ground.
While rotary displacement piling has been around for some time, the innovation in this system is the insertion of casing immediately behind the tapered cone. This allows a conventional bored concrete pile to be installed a further 16m into the chalk beneath.
With all piling due to be complete this month, the technique is being heralded as a major success.
Noise, vibration and ground disturbance have been negligible.
And, even more crucially, there is barely any trace on the surface of the dump's sensitive contents.
'Air and ground monitoring samples contain only negligible concentrations of asbestos dust, ' says Bachy Soletanche construction manager Robin Abraham.
'We have only ever recorded a fraction of our permitted safety levels.'
Factory Farm is the odd name for an inconspicuous tract of land on the eastern banks of the river Medway in Rochester, used for years as a licensed tip for a wide range of domestic and industrial products. It might have remained inconspicuous had alignment for the CTRL's 25 span Medway crossing not clipped the side of the now capped and sealed dump that contained most of the asbestos.
Large quantities had been tipped into bunded cells and spread in 2m layers, sandwiched between similarly thick bands of chalk fill laid as protection.
'Records are incomplete, and we do not have a full inventory of exactly what is down there, but we suspect asbestos in all forms - lagging, boards and sheeting - both loose and in plastic bags, ' Abraham explains. 'Site investigation was initially limited as we were not allowed any coring or direct sampling.'
The £30M Medway Bridge, designed and project managed by Rail Link Engineering, will be supported on 24 solid concrete piers, each with an average of six, 1,200mm diameter piles bored up to 30m into chalk bedrock. Bachy Soletanche, through its £2.5M contract for main bridge contractor Eurolink, last year sank most of the 460 piles.
Prestressed concrete box girder deck sections are already being launched across rows of completed piers.
Only at Factory Farm, the site for four of the bridge's supports, is the neat 1.3km long line of white piers interrupted by piling rigs.
But their continued presence does not signal any alarming contractual delay, for this area has long been regarded as one of the project's main technical challenges, and completion of its piling was deliberately programmed separately to allow for approval of different methods.
Engineers needed the additional design time, as the list of constraints accompanying this narrow, 300m long area was onerous. The presence of asbestos triggered an immediate awareness from RLE and client Union Railways that Factory Farm be treated as a contaminated area: fenced off, controlled access, everyone wearing protective clothing and demanding round the clock air monitoring.
Nothing could be excavated and no arisings from any bored piling brought to the surface. Only sparse site investigation, based on dynamic cone testing, was allowed and even the cones had to be left buried in the ground. But there were further challenges.
Long before the construction teams arrived, this controversial dump was unpopular with local communities. Houses are just 170m away and not only do the environmental constraints include tight noise and vibration limits, but the client was determined not to further concern the residents once piling began.
Twelve, 900mm diameter piles are needed for each foundation and original ideas centred around boring concrete piles through a driven casing.
Discussions between Bachy Soletanche and RLE then moved instead to a more innovative and at the same time more environmentally friendly solution.
The resultant cased displacement piling technique is the outcome of over a year's development and is claimed by the firm to be a world first.
'Not only does it satisfy all the environmental constraints but, in these uncertain ground conditions, it reduces risk to both us and the overall project, ' says operations manager Chris Merridew.
Key to the system is a 2.6m long solid steel, tapering cone with its lower half formed as a helical screw. Using a modified high torque rig, the 1t cone is fixed to the end of the kelly bar and lowered down to the leading edge of a standard pile casing.
As the cone is screwed relatively slowly into the ground, the casing - supported separately by the rig's oscillator ring - follows just millimetres behind. Cone and casing are independent, but are sunk at the same speed (see box).
While the cone displaces the soil sideways, the bore is immediately filled by the casing. Up to three, 9m long tubes are needed to bore through the 20m deep rubbish dump. Temporary removal of the kelly bar and cone - to allow additional casing to be welded on top - is the only time when potentially contaminated soil, stuck to the cone, could be brought to the surface.
The cone is immediately inserted into a water-filled capsule where high pressure jets remove any spoil.Waste water flows into a closed disposal tank. The cone is then returned to the bore to seal the cased section of pile some 2m into chalk.
Once the casing has been installed for all 12 piles needed for each pier, surface soil samples are taken to confirm there is no contamination and allow fencing to be removed. Second stage piling comprises conventional, 36m long concrete bored piles through the casing and founded in the chalk.
With the last piles now being completed there are no signs of either ground contamination or - of near equal concern - any complaints from residents.
'Technically, this solution has reduced the risks all round, ' says Eurolink section agent Steve Meadowcroft, 'and by keeping the public fully informed of what we are doing, we have not received a single letter of complaint.'